Computed tomography (CT) scanning (i.e., using an external X-ray source) and positron emission tomography (PET) scanning (i.e., using an infused radiopharmaceutical as a source of gamma ray emissions) are well known methods for diagnostic medical imaging. CT scanning employs multiple X-ray images taken in multiple directions to generate a 3-dimensional image or multiple tomographic image “slices.” PET scanning employs a gamma-emitting radiopharmaceutical ingested by a patient or injected into a patient. Multiple gamma ray images are taken in multiple directions to generate a 3-dimensional PET image or multiple slices. CT and PET scanning provide different information. For example, CT scanning generally has higher resolution and is superior for providing structural data such as the structure of bones, organs, etc. PET scanning generally has lower resolution but provides more useful information regarding the functional condition of body tissues and systems such as the cardiovascular system. PET is superior for indicating the presence of soft tissue tumors or decreased blood flow to certain organs or areas of the body, for example. The complementary strengths of CT and PET scanning can be provided simultaneously by performing both methods in a single apparatus and imaging session. However, combining CT and PET scanning presents technical challenges because CT and PET require different scan times and have different sensitivities to patient motion.
PET scanning requires a relatively long duration data acquisition period on the order of about 30 minutes for a typical clinically sufficient image. Typically, a large number of PET data acquisitions are acquired at many different angles during this period. Consequently, patient movement is a problem in PET scanning. Excessive motion of a patient can result in scan failure. Thoracic cage movement caused by breathing is a significant problem in PET scanning.
By comparison, CT scanning is relatively fast and can typically be performed during one breath-hold by a patient.
Fusion of CT and PET images often is inaccurate because of inevitable patient movement and breathing. Associated problems include several types of CT artifacts, errors in the association between anatomy and PET uptake, motion blur in PET, and quantitative errors PET errors such as miscalculation of the standard uptake value due to underestimation or overestimation of attenuation.
Part of the solution to these problems is to provide gating of PET and CT scanning based on measurement of certain triggering parameters associated with respiratory motion. In particular, it is known in the art to use a strain gauge to measure the tension in a strap placed around the abdomen or chest of a patient. Signals from the strain gauge are used to develop information that can be used to gate or trigger the operation of imaging apparatus.
The accuracy of such approaches is limited by the fact that patient breathing patterns change over the time period involved in performing the diagnostic scan. This problem is illustrated in the strain gauge traces of FIG. 1. The traces show that deep, irregular breathing at one point in time can be followed by a more regular, shallower breathing pattern ten minutes later. In FIG. 1, the horizontal axis represents time, with a one minute interval between the left and right sides of each plot. The vertical axis represents the signal from the strain gauge, with smaller values corresponding to a more relaxed chest and larger values corresponding to a more expanded chest. Thus, trigger signals based on respiratory cycle alone do not solve the misregistration problem.
Accordingly, there is a need in the art for improved methods for combined CT and PET scanning. It would be particularly beneficial to provide a method for combined CT and PET scanning that can correct for inaccuracies caused by patient motion such as motion caused by respiration.